Solar thermal collector

ABSTRACT

The invention relates to a solar thermal collector for heating a fluid to be heated ( 600 ), comprising:—a thermally insulating body ( 1 );—a light transparent barrier ( 2 );—a heat accumulator ( 3 ) comprising a thermal accumulating material ( 30 ), and—a heat exchanger ( 5 ) designed for transmit thermal energy from the thermal accumulating material to the fluid to be heated ( 600 ), wherein heat exchanger ( 5 ) is formed by a pileup of die-forged metal sheets and wherein the thermal accumulating material consists of a phase transfer material comprising a salt solutions based hydrogel and gelling agents.

TECHNICAL FIELD

This invention relates to solar thermal technology, in particular to theequipment designed for the conversion of solar energy into thermalenergy, and can be used for heating water, especially residential orindustrial water.

BACKGROUND ART

It is known a solar thermal collector, consisting of a body in the formof a water cylinder with light-transparent (translucent) glazing andlight-absorbent coating (see for example document RU 2′108′520).

Such collector has the following drawback: water is heated by the sunthrough a light-absorbent coating but it lacks heat accumulationcapacity.

It is also known from the state of the art a solar thermal collectorconsisting of an insulated body with a lid comprising alight-transparent glazing and a corrugated inner wall. The area betweenthe base of the body and the corrugated wall is filled a heataccumulating material in the form of a phase transformation material(for example paraffin). Water is supplied into the thermal energycollector via a flow heat exchanger (for instance a heater coil) so asto exchange thermal energy with the thermal accumulating material (seefor example documents RU 2′230′263 and CN 101285622).

In reference to FIG. 7, a thermal collector according to the state ofthe art comprise the following important features: a thermallyinsulating body 1, flow heat exchanger 5 (to heat the water), alight-transparent glazing 2, a selective light absorption material 4, aheat accumulator 3 in the form of a phase transformation material, flowpipes 6 for water circulation and thermal conductive elements.

However in the mentioned existing solar thermal collector solar energy Ris first absorbed by the water. When there is too much sun, the excessenergy heats and melts the phase transformation material (heataccumulator 3), i.e. thermal energy is accumulated. When there is nosun, the water is heated by the thermal energy radiated when the phasetransformation material crystallises.

According to another embodiment, solar energy R is first absorbed by theselective light absorption material which then transmits thermal energyto the phase transformation material. Metallic ribs inside the thermalcollector allow the transition of the thermal energy from the phasetransformation material to the water enhance the thermal transmission.

Furthermore heat accumulating material as paraffin does not have theability to keep the thermal energy for a long time and loses thermalenergy by thermal transmission, convection and emission.

It also has to be noted that calculation of the volume of the tank isbased on the condition that overheating of the water collected in thetank is not permitted, i.e. the water must not be heated to 100° C. orabove. In other words, the volume of the tank is calculated from themaximum value of energy received by the heater from the heat source (anintensity index is introduced), taking into account the practicalspecific heat of water and the maximum allowed temperature, 100° C.

An example of solar collector calculation follows. For an area with amaximum solar insulation per day of 17 MJ/m², the capacity of acollector equals: 17,000,000 J/4190 J/kg/K/80 g×K=50.75 l/m².

This implies that, if the efficiency of the solar collector were 100%,one square metre would be sufficient to heat 50.75 l of water to 80° C.Consequently, the tank must hold at least 50.75 l of water per one metresquare of the absorbing surface of the solar collector. In practicehowever, efficiency of solar collectors is no greater than 40-70%.Consequently, a solar collector with an absorbing surface area of 2 m²requires at least 71.5 l heat accumulation tank.

To attract buyers, that volume is usually increased to 100-150 l. Theresult is obvious: the larger volume of water in the tank requires moreheat; therefore either the water is not heated to the requiredtemperature or an additional heat source is required (a gas or electricheater for example). On the other hand, even on maximal conditions (hotand sunny days), when the water does reach the required temperatureheated only by the sun, the customer cannot use that volume of heatedwater efficiently because hot water is discharged from the collectingheater by displacing an equal volume of cold water, i.e. hot water mixeswith cold water. Thus discharging just 30 l of water heated to 58° C.from a 100 l collector and replacing it with tap water at 20° C. resultsin a temperature drop to 46.6° C. The use of the next 30 l results inthe temperature dropping to 42° C.

To eliminate this drawback, manufacturers resort to various structuraldevices, such as a layer-by-layer filling, installing additional tanksinside the main one (to achieve convective heat exchange), etc.

However the following drawback is shared: heat is accumulated with theincrease of the internal energy of the material (heating the water)during the heating process. In other words, energy accumulation dependslinearly on the temperature of the material.

In addition to this major drawback, the collectors according to thestate of the art suffer precipitation of insoluble salts from the waterbecause the conditions inside boilers are favourable to the growth ofcrystals, creating a serious bacteriological problem: colonies ofharmful bacteria develop in the warm, porous deposits on their walls, .. . .

Also, these deposits are responsible for a dramatic drop of heatexchange efficiency: efficiency drops, while energy consumptionincreases. If the tank is not flushed regularly, water in it mightoverheat. This will cause the increase of pressure in the tank and theprimary heat carrier supply pipes, which often results in failures. Onlyhaving their structure cardinally altered can eliminate these genericproblems of accumulating heaters.

SUMMARY OF INVENTION Technical Problem

It is an object of the present application to provide a novel solarthermal collector avoiding those drawbacks.

More particularly, an object of the present invention is to improve thedesign of flow solar thermal collector for reducing heat losses duringheat accumulation and water heating, thus improving efficiency of thesolar energy use.

Solution of the Problem

This object is achieved by a solar thermal collector according toannexed claim 1.

Advantageously but optionally the present invention comprises at leastone of the following features:

-   -   at least one surface of the heat accumulator is coated with a        selectively light-absorbing material,    -   the thermal accumulating material comprises a sodium acetate        solution in distilled water (acetate trihydrate) with a gelling        agent,    -   the gelling agent comprises a solution of carboxymethyl        cellulose (CMC) and/or a solution of polyvinylpyrollidon (PVP)        and/or a solution of sodium laureth sulphate and/or carrageenan,    -   the thermal accumulating material comprises a coating of high        coefficient thermal dilatation material,    -   the high coefficient thermal dilatation material comprises        paraffin.

Advantageous Effects of Invention

The present invention is essentially based on the combination of saltsolutions based hydrogel and gelling agents as phase transformationmaterial and thermal conductive die-forged metal sheets that are piledup in order to constitute a heat exchanger means.

The solar thermal collector according to the present invention ensures:

-   -   firstly that the surface of the heat accumulator absorbs        sunlight with high efficiency, which results in that a greater        amount of thermal energy is transferred to the phase        transformation material;    -   secondly it improves the convection heat transfer between the        heat accumulator and the elements of the flow heat exchanger.

As a result, heat losses are reduced and it allows efficient thermalcollector.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 a represents a schematic overview of the solar thermal collectoraccording an embodiment the present invention;

FIG. 1 b represents lateral cross-section of solar thermal collector ofFIG. 1 a;

FIGS. 2 a and 2 b represent a solar thermal collector inner structureaccording to a specific embodiment of the present invention;

FIGS. 3 a, 3 b and 3 c represent a solar thermal collector innerstructure according to a specific embodiment of the present invention;

FIGS. 4 a and 4 b represent a solar thermal collector according toanother embodiment of the present invention;

FIG. 5 represents a solar thermal collector designed to be put in awindow according to a specific embodiment of the present invention;

FIG. 6 represents a solar thermal collector designed to be put on a roofaccording to a specific embodiment of the present invention;

FIG. 7 represents a solar thermal collector according to the state ofthe art.

DETAILED DESCRIPTION

In reference to FIGS. 1 a and 1 b, the collector C of solar thermalenergy according to a particular embodiment of the present inventionincludes a thermally insulated body 1 with a light-transparent glazing2.

The body 1 comprises flow pipes 6 and more particularly an inlet pipe 60and an outlet pipe 62 feeding the solar thermal collector C with fluidto be heated as water. More particularly, the fluid to be heated entersinto the solar thermal collector body 1 through pipe 60, is heatedinside the body 1 and then exits trough pipe 62. It can be used in adomestic or industrial field.

A heat accumulator bloc 3 is arranged inside the body 1. The heataccumulator bloc 3 is filled with thermal accumulating material 30.

The heat accumulator 3 comprises a selective light absorption coating 4.The heat accumulator is in cooperation with a flow heat exchanger 5troughs which the fluid to be heated flows.

The coating 4 is made of a material with high absorption coefficient andlow reflection coefficient (blackened copper might be used for exampleor niello copper).

The heat accumulator 3 is hydraulically connected to flow pipes 6 (inletpipe 60 and exit pipe 62).

The heat exchanger 5 is designed to provoke a thermal energy exchangebetween the phase transformation material 30 and the fluid to be heated600 (e.g. the water) flowing inside the heat exchanger 5.

In reference to FIGS. 2 a and 2 b and according to a specific embodimentof the present invention, the heat exchanger 5 comprises a corestructure 54 for the thermal energy exchange between the fluid to beheated 600 and the thermal accumulating material 30.

The heat exchanger core structure 54 is formed with die-forged metalsheets 540 which are piled up.

All the sheets 540 of the heat exchanger core structure 54 areidentical. Each sheet 540 comprises deformations obtained in adie-forged process.

When the sheets 540 of the heat exchanger core structure 54 are piledup, the deformations of the sheets form closed compartments 560 of aglobal channel 56 receiving for the fluid to be heated (e.g. the water)and being hermetically sealed by hermetic seal channels 58. When thesheets 540 are piled up it also forms cavities 57 that are designed toreceive the accumulating material 30. In this embodiment, the channel 56forms a straight longitudinal channel along the piled-up sheets. Eachsheet 540 comprises a hole 542 located along this channel 56.

In reference to FIG. 2 b the hole 542 of a given sheet is located in theopposed extremity of the channel 56 than the hole of the subsequentsheets.

For instance the hole of sheets 540 a and 540 c (holes 542 a and 542 c)are located on the left side of the channel 56 and the hole of sheets540 b and 540 d (holes 542 b and 542 d) are located of the right side ofthe channel 56.

As a consequence, the water begins to flow by hole 542 a, enters intothe channel compartment 560 located between sheets 540 a and 540 b, thenflows along the channel compartment until hole 540 b, then flows throughthe hole to arrive inside the channel compartment located between sheets540 b and 540 c.

And so on until the water has circulated inside each channelcompartments until the exit after the hole 542 d.

In reference to FIGS. 3 a, 3 b and 3 c and according to anotherembodiment of the present invention, the heat exchanger 5 comprises aninlet 50 linked to the inlet pipe 60 of the solar thermal collector andan outlet 52 linked to the outlet pipe 62 of the solar thermalcollector.

A channel 56 inside the heat exchanger 5 links the inlet 50 to theoutlet 52 along which the thermal energy exchange is performed betweenthe fluid to be heated 600 is heated and the thermal accumulatingmaterial 30.

The heat exchanger 5 comprises at least one channel 56 for the flow offluid 600 to be heated (as water) and cavities 57 through which it isheated by the heat release by the thermal accumulating material 30inside the heat accumulator 3.

The heat exchanger 5 comprises a core structure 54 for the thermalenergy exchange between the fluid to be heated 600 and the thermalaccumulating material 30.

The heat exchanger core structure 54 is formed with die-forged metalsheets 540 which are piled up.

All the sheets 540 of the heat exchanger core structure 54 areidentical. Each sheet 540 comprises a deformation 5401, preferably in ahalf-pipe form groove. This groove is along a path following thetrajectory 5042 of the channel 56.

When the sheets 540 of the heat exchanger core structure 54 are piledup, the deformation 5401 form closed compartments 560 of the channels 56receiving the fluid to be heated (e.g. the water) and being hermeticallysealed by hermetic seal channels 58.

The areas outside of the fluid channel 56 (and seal channel 58) formcavities 57 to be filed with thermal accumulating material 30.Accordingly cavities 57 and channels 56 are separated each from other byhermetic seal channels 58.

Sheets 540 material of the heat exchanger 5 represents the heat transfermedium between the thermal accumulating material and the fluid to beheated.

This heat exchanger core structure 54 allows building a one-flow or amulti-flow heat exchanger of any kind: liquid-solid, liquid-liquid,liquid-gas, gas-solid etc.

Moreover this heat exchanger core structure 54 comprises single standardcomponents (sheets 540) and contains no soldered or welded joints. Thusit is highly suitable for computerised/automated manufacturing.

Furthermore maintaining and repairing such heat exchanger core structure54 is easy as sheets 540 can be dismantled and re-assembled severaltimes without resorting to either soldering or welding equipment.

All the channel compartments 560 formed between each sheets 540 of theheat exchanger core structure are fixed by a fixing means as a singlethreaded bush (not shown) that draws together all the sheets into asingle block.

A by-pass valve (not shown), either rotating around the rotation axis ofthe bush or longitudinally-sliding along that axis can be installedinside the bush, enabling to switch the channels (there is also room fora small turbine-oscillator for agitating sound waves in the fluid to beheated).

In addition to the bush, all the sheets 540 are drawn together by pins59. The required capacity of the at least one channel 56 is achieved bydesigning it accordingly: a gap between the sheets is controlled bygaskets (not shown).

In addition, the channel compartments 560 have dome-shaped section,which imparts to them excess resilience in both longitudinal andcrosswise directions. Accordingly the assembly of the sheets 540 pullsthem together very tightly.

Hermetic sealing of the channel compartments 560 can be enhanced by theusing of a fluoro-rubber, butyl, latex, or silicon compound coating,placed into the grooves hermetic seal channels 58 running along thefluid channel 56.

In a first embodiment the water flowing from the inlet 50 and enters inthe first compartment 560, circulates along the zigzag channel 56 andonce arrived at the opposed hole, the water flows into the subsequentchannel compartment in the same manner than previously explained (exceptthat the channel 56 has a zigzag form).

Alternatively the flowing of the water from inlet 50 to outlet 52 can beparallel inside all the channel compartments 560.

In order to from the heat accumulator 3, the assembled heat exchangerblock 5 is placed in a hermetically sealed body 1 filled with a thermalaccumulating material 30.

The Z-shaped channel forms a hydraulic seal to eliminate the possibilityof the air breaking into the heat exchanger when the fluid is dischargedfrom the heat exchanger ('totally air-locked').

Concerning the pipes 6 the air-lock principle has been applied:

-   -   a height difference must be ensured in the flow heater,        installing it at an angle to the horizon to let the air out when        the heat exchanger is being filled with water,    -   a by-pass valve—to turn off water and to force out the left-over        water from the heat exchanger, and    -   a wash ramp: a pipe with sprayers for cleaning the glass of the        solar accumulator.        Water is let into the heat exchanger only when the customer        opens the supply valve. When the valve is shut, water is        discharged to the customer. The length and cross section of the        discharge are selected so as to ensure that leftover fluid is        sucked out from the heat exchanger by gravity when the valve at        the uppermost point in the system is open. Water is supplied        either through the vent on the supply main—the simplest case—or        via a special slide valve,

In addition to being influenced by gravity, the leftover fluid in theheat exchanger expands during the heating, i.e. the water is ‘squeezedout’ through the gaps in the heat exchanger. Having the smallestpossible space inside the channel is a necessary requirement forspeeding up heat transfer and minimising the heat loss.

The thermal accumulating material 30 is basically a hydrogel and gellingagents as phase transformation material.

Particularly, the thermal energy accumulation happens in the form ofsalt dissolution in crystallisation water and fixing (envelopment) ofthe water residues by the gelling agents.

Preferably the thermal accumulating material 30 is a eutectic mixtureand preferably a sodium acetate hydrate gel can be used as the phasetransfer material.

Such thermal accumulating material quickly reaches its melting pointwhen heated. Using this type of material has the following advantageover continuous, linear heating: it receives and releases energy whenthe material undergoes the liquid-crystalline phase transformation whilethe temperature remains constant.

A molten crystalline material, for example, releases 60-80% of itsuseful heat while its temperature does not drop. In other words, withsuch thermal accumulating material, the first 60-80% of the total volumeof the fluid to be heated by the thermal accumulating material is heatedat a constant temperature.

The temperature begins to drop only after crystallisation is completed.Consequently, the amount of the thermal energy used does not affect theefficiency of this accumulator.

The thermal energy release (because of the salt crystallisation) istriggered by an external solicitation which can be mechanical orelectrical on the gel.

In comparison to the prior art embodiment using the paraffin, thepresent invention uses the dissolution process of the salt in a solvent,as crystallisation water. Accordingly the thermal accumulation materialdoes not “melt” during the temperature increase but “dissolves” (intoions in the electrolyte).

The hot dissolution of the salt crystals is stabilised by the gellingagents when the hydrogel is formed.

Thus the gelling agents have the following functions:

envelopment of all additive elements,

avoiding convection phenomena,

absorption of mechanical oscillation,

Accordingly the emergence probability of a crystallisation centredecreases.

In the event gel is subjected to a violent shock (ultrasound,cavitations, electrical . . . ), salt begins precipitation in thesaturated solution. This phenomenon is exothermic.

In a specific embodiment, the hydrogel is cover by a coating of highcoefficient thermal expansion material whose melting temperature ishigher than the dissolution temperature (e.g. the paraffin). This kindof material decreases in volume when the temperature decreases and thuscompensates for the volume increase due to the crystallisation.Therefore mechanical tensions inside the heat accumulator are reduced.Preferably the high coefficient thermal expansion material volumerepresents 5-10% of the total volume.

In a preferred embodiment, a super-saturated aqueous solution of sodiumacetate is used as a thermal accumulating material.

A saturated solution has the following advantage over solids: when itstemperature decreases, the solubility decreases, which means that it isable to form a “super-cooled” (‘chilled’) molten salt, dissolved in afluid that will release its melting heat during recrystallization.

Sodium acetate solutions can be “super-cooled” in a range from 50° C. to60° C. (generally by 52° C.) without releasing the accumulated thermalenergy. Consequently, it is possible to provide a thermal energyaccumulator material that stores the energy not due to intensive thermalinsulation (like with a thermal flask) but due to a phasetransformation, and thus releases the accumulated thermal energy whenrequired.

The dissolution point of commercial sodium tri-hydrate acetate is in therange of 50 to 60° C. and generally 58° C.

Sodium tri-hydrate acetate is not as corrosive as other salts.Accordingly using this salt enables to build a compact heat accumulatorwhile maintaining good capacity.

To ensure stable thermo-physical properties, a special hydrogel (alsocalled “aquagel”) can be used: a sodium acetate solution in distilledwater (acetate trihydrate) with a gelation agent: a weak solution ofcarboxymethyl cellulose (CMC) and/or of polyvinylpyrollidon (PVP) and/orsodium laureth sulphate and/or carrageenan.

In a preferred embodiment, the following ratio is used:

Hydrogel composition (mass percent):

sodium acetate trihydrate  96% CMC 700 3.0% PVP 1.0%The dissolution/crystallisation heat of this gel is 282,000 J/kg, whileit thermal capacity goes from 2,650 to 2,800 J/kg/° C.

The mixing process has been verified with a weak solution (0.001% bymass) of phenolphthalein, an acidity indicator. The thermal accumulatingmaterial itself did not overheat when the heat accumulator wasre-loaded. It neither boils nor causes explosion because the boilingtemperature of this hydrogel is far above 100° C., while the point ofequilibrium between the heat inflow and heat losses by radiation in thesolar collector lies in the 90 to 96° C. interval.

To allow the expansion of the gel due to the thermal increase, the heataccumulator is designed with spare capacity.

In a preferred embodiment of the present invention, 10% of the spaceoccupied by the heat accumulating material is provided as sparecapacity.

Using a eutectic mixture, a sodium acetate hydrate gel for example, asthe phase transformation material, reduces the amount of energy requiredto melt the phase transformation material because the meltingtemperature of a eutectic mixture is lower than the melting temperatureof a mixture of any other composition; this also results in thereduction of heat loss.

The charging-discharging of the heat accumulator follows a certainsequence: a rapid temperature increase and stabilisation during thecharging stage and, vice versa, a long plateau of discharge temperature,which does not require any additional control or stabilisation.

The discharge of the heat accumulator 3 includes two phases:

-   -   the thermal charging when salts are dissolved and the mixture        supercools;    -   the thermal discharge due to the re-crystallization of the        eutectic mixture at any moment chosen by the customer with a        mechanical or electrical trigger as previously explained.

A solar thermal collector according to the present invention with directheat absorption allows the most efficient use of solar energy. Applyingthe sun radiation absorbing layer directly on the surface of the heataccumulation block sets ideal conditions for the storage of solarenergy:

First of all, the present invention has eliminated various auxiliarydevices, primary lines used in other designs, so that thermal energy isdirectly transmitted to the accumulating material 30.

Secondly, heat transfer has two stages:

-   -   in the first stage, the accumulating material is simply heated        until its melting starts (salts dissolution). Since the mass of        the accumulating material is smaller than that of a water boiler        of a similar capacity, while its heat capacity is about half of        that of the latter, the accumulating material 30 heats up three        times faster than a similar water boiler, which is also more        heavily insulated.    -   During the second stage the accumulating material melts, while        its temperature stays practically constant. This makes the solar        thermal collector even more efficient because its losses through        radiation are lower and also because its functioning in        changeable cloudiness conditions i.e. the influx of thermal        energy is not affected by the temperature variation due to the        direct heat transfer from the selectively absorbing layer.    -   Thirdly, the heat transfer is carried out without any auxiliary        electrical or mechanical devices, such as circulation pumps,        heat syphons, etc., which makes its work extremely reliable        throughout the entire process.        Advantageously the absorbing layer is protected by a guard made        of super-transparent borosilicate glass. The guard can be made        in the form of a one- or multi-chamber pack of glass.

The thickness of the assembled heat exchanger block has been set on thefollowing basis: the thermal accumulating material 30 needs to bemelted. Consequently, the hydrogel layer (accumulating material 30) mustbe thick enough to let in a quantity of solar energy sufficient for themelting the entire volume of the thermal accumulating material 30 on anaverage sunny day—given a particular surface area. Consequently, themass of the heat accumulation material 30 needs to be from 30 to 70kg/m² of the absorbing surface, depending on the average-annualinsulation of the location where the solar thermal collector is going tobe used.

Three different accumulators have been tested while respecting theweight per unit surface: 60, 45 and 30 mm thick.

Also, to improve the range of appropriate insulation levels in anaverage size solar accumulator, the author decided to assemble threeblocks of accumulators of different thickness, connected in series: thethickest one nearest to the water supply, then the average, then thethinnest one.

The advantage of this distribution over an accumulator consisting ofthree identical block is that even when sun energy is very small,insufficient for heating through the thick and heavy blocks, the thinblocks will still store some energy, sufficient to heat up a smallamount of water. On the other hand, even the thickest blocks of greatcapacity will still transfer some of its energy to the water, althoughat lower temperatures, while the thin, hotter blocks will further heatthe water.

The thermal insulation of the solar thermal collector includes thethermal insulation of the accumulator blocks and the insulation from theatmosphere on the absorbing surface side.

The insulation guard of the solar thermal accumulator can be of any typeas a vacuum cavity.

In the preferred embodiment of the invention, the insulation consists ofa glass made of pure glass with silica gel in the distance controlframe.

To reduce the heat losses through radiation in the infrared range, thetransparent guard is provided with an inner layer of infrared mirror.Also the transparent guard is designed with two-glass plate glass packsin order to reduce convection losses.

For example, an infrared mirror can be made by gluing some special TC-88film manufactured by the 3M Company to the glass inner surface, or bydepositing a thin layer of indium oxide, using a vacuum ionic device.

It is also possible to use a packing between the accumulator block andthe distance control frame of the glass, manufactured from a compositematerial able to insulate the glass from the hot absorbing surface.

According to a specific embodiment of the present invention, thismaterial is manufactured by the impregnation of sheets of very thinbasalt fibres (no greater than 2 μm in diameter) with liquid ceramicinsulation. A free area is left between the distance-control frame andthe body, which runs along the perimeter of the glass. This area ispainted with an absorbing black paint (a primer) on the inside.

The collector according to a specific embodiment of the presentinvention also comprises heat-conducting elements to conduct heat fromthe selectively light-absorbing coating to the phase transformationmaterial and optionally to the water in the heat exchanger. For examplethe heat-conducting elements are in the form of ribs. To eliminate heatlosses by thermal radiation through the bottom surfaces of the ribs, thelatter are insulated with a special composite coating on the inside.Furthermore the ribs can be installed in a network of the glass fibredipped in liquid ceramic thermal insulation material as previouslydescribed.

As another measure designed to reduce convection losses: a heated buttof the glass absorbs less heat from the inner air layer, which preventsconvection.

The thermal insulation of the accumulator includes three stages:

-   -   first of all it is the body of the accumulator block itself,        which is manufactured from polished metal and works like a        mirror for infrared thermal radiation.    -   Secondly, the ‘liquid ceramic’ layer deposited onto the        accumulator blocks, represents a specialised high-temperature        insulation (for example “Astratech”® or similar products of the        domestic industries can be used). The heat resistance of these        materials is very high.    -   The third stage is formed of polyurethane foam, which, similar        to sandwich structures, also works as a structural component,        keeping together the outer casing and the inner blocks.        The body of the solar collector withstands considerable        stresses: heat distortion, atmospheric precipitation,        transportation stresses. Therefore, according to a preferred        embodiment of the present invention, the body is made of        polyvinylchloride.        Alternatively, vacuum-formed shell bodies from thermoplastic        materials or glass-reinforced plastics, based on acrylic or        epoxy resin can be chosen. Indeed such features are commonly        widespread in the industry.        Also shell bodies with a polyurethane packing, sandwich        structures, possess high mechanical and impact strength, are        heat and frost resistant, cheap and not heavy. Advantageously        the transparent guard is secured to the body by being glued with        a polyurethane glue for glass at the special notches (for        example “Teroson”®). The distance control frame is glued between        the accumulator block and the glass by the means of a butyl heat        resistant glue. This allows for some movement and glass        vibration and protects the glass from fracturing.

Referring to FIGS. 4 a and 4 b and according to another specificembodiment of the present invention, the thermal accumulator furthercomprises thermal conductive elements 7 for example in the form of metalbars 70 comprising ribs 72 (preferably in form of louvers) thermallylinked to the metal bars 70. Thermal conductive elements 7 are thermallylinked and are preferably joined into a pack, made of heat conductingmaterial, metal for example.

The solar energy collector operates as follows. Sun rays reach theabsorption louvers 72 and heats them. Thermal energy is transferred fromthe louvers 72 to the metal bars 70 that bring further thermal energy tothe collector.

In a specific embodiment, the metal bars 70 can be filed with any fluidable to efficiently transport thermal energy. In a specific embodimentof the present invention, the fluid is an evaporating fluid. Accordinglywhen the evaporating fluid is heated by the absorption of solar energy(metal louvers also allow the absorption and the conduction of thethermal energy to the metal bars 70) the evaporating fluid begins toevaporate, raises inside the metal bars and then reach the extremity 700of the bars 70 that terminally linked to the collector.

This solar thermal collector has the following advantages over the solarenergy collectors according to the state of the art:

-   -   It can be installed in an existing window frame W (as        represented in FIG. 5). Such window lets into the house some of        the reflected and of the scattered light, acting not only as a        heater but also as an ordinary window.    -   According to FIG. 6, a solar thermal collector according to the        invention can also be installed at an angle frame, incorporated        into the roof for example. It can be also used as skylight        windows.

1. A solar thermal collector for heating a fluid to be heated (600),comprising a thermally insulating body (1), a light transparent barrier(2), a heat accumulator (3) comprising a thermal accumulating material(30), and a heat exchanger (5) designed for transmit thermal energy fromthe phase transfer material to the fluid to be heated (600), whereinheat exchanger (5) is formed by a pileup of die-forged metal sheets andwherein the thermal accumulating material comprises a salt solutionsbased hydrogel and gelling agents.
 2. A solar thermal collectoraccording to claim 1, wherein at least one surface of the heataccumulator (3) is coated with a selectively light-absorbing material(4).
 3. A solar energy collector according to claim 1 or 2, wherein thethermal accumulating material comprises a sodium acetate solution indistilled water (acetate trihydrate) with a gelling agent.
 4. A solarenergy collector according to any of claims 1 to 3 wherein the gellingagent comprises a solution of carboxymethyl cellulose (CMC) and/or asolution of polyvinylpyrollidon (PVP) and/or a solution of sodiumlaureth sulphate and/or carrageenan.
 5. A solar energy collectoraccording to any of claims 1 to 4 wherein the thermal accumulatingmaterial comprises a coating of high coefficient thermal dilatationmaterial.
 6. A solar energy collector according to claim 5, wherein thehigh coefficient thermal dilatation material comprises paraffin.
 7. Asolar energy collector according to any of claims 1 to 6 wherein itfurther comprises metal bars (70) thermally linked to thermallyinsulating body (1).
 8. A solar energy collector according to claim 7wherein it further comprises metal ribs (72) thermally linked to themetal bars (70).
 9. Window comprising at least a solar energy collectoraccording to claim 8 wherein the metal ribs (72) are in the form oflouvers.